1. Field of the Invention
The present invention relates to a color image reading apparatus, and more particularly to a color image reading apparatus provided with color separation means comprising a one-dimensional reflective blazed diffraction grating, and light receiving means including three line sensors (photosensors) mounted on the same substrate, capable of preventing image blur on the photosensors resulting from aberration in the reflective diffraction angle caused by a difference in the incident light angle thereby enabling to read color image information of an original image with a high precision, and adapted for use in a color scanner, a color facsimile apparatus or the like.
2. Description of Related Art
There have been proposed various apparatuses for focusing the color image information of an original image onto line sensors (CCD) through an optical system and reading the color image information in digital manner, utilizing the output signals of said line sensors.
As an example, FIG. 1 schematically illustrates the optical system of a conventional color image reading apparatus, wherein the light from a color image on an original image plane 11 is condensed by an imaging lens 19, then split into three colors, for example red (R), green (G) and blue (B) by a 3P prism 20, and guided to respective line sensors 21, 22, 23, and the color images focused on said line sensors are respectively read by scanning in the sub-scanning direction.
FIG. 2 also illustrates schematically the optical system of another conventional color image reading apparatus, wherein the light from a color image on the original image plane 11 is condensed by an imaging lens 29, and is split into three light beams corresponding to three colors, by beam splitters 30, 31 with dichroic transmission films, and said three color beams are respectively focused on three line sensors, formed oh so-called monolithic 3-line sensor 32 and are respectively read by scanning in the sub-scanning direction.
FIG. 3 is a schematic perspective view of the monolithic 3-line sensor 32 shown in FIG. 2. Said sensor 32 is provided with three line sensors (CCD's) 25, 26, 27 arranged in a mutually parallel manner and with a certain mutual distance, on a same substrate, and said line sensors are respectively provided thereon with unrepresented color filters corresponding to the colors of the light beams.
The distances S1, S2 of said line sensors 25, 26, 27 are generally in a range of 0.1-0.2 mm based on various conditions of preparation, and the pixel sizes W1, W2 of each single element is for example in a range of 7.times.7 to 10.times.10 .mu.m.
The color image reading apparatus shown in FIG. 1 is complex and expensive as it requires not only three independent line sensors but also a 3P prism which necessitates a high precision and is difficult to manufacture. Also the assembling and adjustment are cumbersome since the alignment between the light beams and the line sensors has to be made three times independently.
Also in the color image reading apparatus shown in FIG. 2, the distance between the line sensors is represented by 2.sqroot.2t, for a plate thickness t of the beam splitters 30, 31. Thus, for a distance of 0.1 to 0.2 mm, which is preferred for the manufacture of line sensors, the plate thickness t for the beam splitters 30, 31 has to be in a range of 35 to 70 .mu.m.
It is in general very difficult to produce a beam splitter of a satisfactory optical flatness with such a small thickness, and the color image focused on the line sensors becomes inevitably deteriorated when beam splitters of such a small thickness are employed.
On the other hand, in the monolithic 3-line sensor, the distances S1, S2 of the lines 25, 27 from the central line 26, as shown in FIG. 4, are generally selected as mutually equal and as an integral multiple of the pixel size W2 (cf. FIG. 3) in the sub-scanning direction, for the following reason.
In case of reading a color image with the above-mentioned monolithic 3-line sensor through an ordinary imaging optical system 45 as shown in FIG. 4, three line sensors 25, 26, 27 read three different positions 25', 26', 27' on the original image plane 11 at the same time.
Stated differently, the three-color (R, G, B) signal components of a given position on the original image plane 11 cannot be read simultaneously but have to be obtained by time adjustments after signal reading with three line sensors.
More specifically, three color signal components can be relatively easily obtained by selecting the distances S1, S2 of the three line sensors as an integral multiple of the pixel size W2, and for example delaying the G, R signals (corresponding to the lights of green and red colors) with respect to the B signal (corresponding to the light of blue color) by means of redundant line memories.
For this purpose, the distances S1, S2 of the line sensors 25, 27 from the central line sensor 26 are selected as an integral multiple of the pixel size W2 in the sub-scanning direction.
However, in such a color image reading apparatus, for providing the redundant line memories corresponding to the gaps in the three line sensors, there are required plural units of expensive line memories. Such configuration is extremely disadvantageous in cost, and also complicates the entire device.
Also there is known, as disclosed in the U.S. Pat. No. 4,277,138 a color image reading device employing a blazed diffraction grating as the color separation means, instead of dichroic mirrors.
However, the disclosed configuration is only designed for the light coming from a point in the object, and does not consider so-called image angle characteristics arising from a fact that the object has a finite reading width in the main scanning direction.